Thursday, January 5, 2012

Chemistry Thursday: Isomers

I mentioned in the post on lavender essential oil that there were two stereoisomers of linalool - what the heck is an isomer?

I liked this family tree type picture to give you an idea of the different kinds of isomers, so I'm including it in this post!

Isomers are compounds that have the same molecular formulas, but have different structural formulas. You can see in this picture that we have 7 hydrogen, 3 carbon, and 1 fluoride in each molecule - which means we could write the molecular formula for either as H7C3F -  but flouride is connected in a different place in each compound. Although they have much in common, these constitutional isomers don't necessarily have the same chemical properties and aren't necessarily very similar in nature. The placement of that F is very important because it means the entire molecule could be different.

Take a look at this picture of a fatty acid. If the double bond is on the 3rd carbon from the end, then we have an omega-3 fatty acid. If the double bond is on the 6th carbon from the end - and only the 6th carbon - then we have an omega-6 fatty acid. If the bond is on the 9th carbon from the end, we have an omega-9 fatty acid. The difference between these three fatty acids is where the double bond exists, so they have the same molecular formula but have very different physical properties, like boiling points, melting points, cloud points, rates of oxidation, and so on. These fatty acids would be isomers of the other because they have the same molecular formula but a different physical construct. We can look at an omega-3 and omega-6 fatty acid and know that although they have the same number of carbon and hydrogen atoms, one might have a shelf life of a month and have a boiling point of 155˚C while the other might have a shelf life of a year and have a boiling point of 380˚C. (I'm making up these numbers to illustrate my point!)

"Stereoisomers are isomeric molecules that have the same molecular formula and sequence of bonded atoms (constitution), but that differ only in the three-dimensional orientations of their atoms in space." So stereoisomers have the same molecular formula and the bond connections are the same, "but their orientation in space differ." (Wikipedia)

"Enantiomers are two stereoisomers that are related to each other by a reflection: They are mirror images of each other, which are non-superimposable....They have the same physical properties, except for the direction in which they rotate polarized light and how they interact with different optical isomers of other compounds. As a result, different enantiomers of a compound may have substantially different biological effects." (Wikipedia) In other words, they have the same molecular formula, but differ in their structure by being reflections of the other. As you'll see in the next example, they might look very similar but we can have different reactions to them!

In the post on linalool, we noticed that there are two stereoisomers of linalool - licareol or (R)-(-)-linalool found in lavender, bay laurel, neroli, bergamot, and sweet basi,l and coriandrol or (S)-(+)-linalool found in coriander, palmarosa, petitgrain, and sweet orange flowers.

We also noticed that one stereoisomer had an R and - in front of the name and the other had an S and + in front of its name. These letters/symbols indicate that these are enantiomers. The R and S designation are based on the Cahn–Ingold–Prelog priority rules (CIP), based on atomic number. (Click here to read more about how these are named. It's a bit detailed for this post!) The + and - in the name relates to optical activity. "An enantiomer can be named by the direction in which it rotates the plane of polarized light. If it rotates the light clockwise (as seen by a viewer towards whom the light is traveling), that enantiomer is labeled (+). Its mirror-image is labeled (−). "

You might hear these molecules being described as chiral - a molecule that "lacks an internal plane of symmetry and thus has a non-superimposable mirror image." (Wikipedia)

And if you want to know more about naming chiral molecules, click here

What does this all mean? These enantiomers can look quite similar but taste and smell quite different! You can see that the (R)-(-)-linalool smells like lavender but the (S)-(+)-linalool smells floral. "Spearmint leaves and caraway seeds, respectively, contain L-carvone and D-carvone - enantiomers of carvone. These smell different to most people because our olfactory receptors also contain chiral molecules that behave differently in the presence of different enantiomers." (Wikipedia) These limonene enantiomers have different effects on us: The D-limonene has a clean, citrus-y odour, whereas the L-limonene has a piney, turpentine-like odor.

If you want to learn more about isomers and chirality, click on this WikiBooks link for a much more detailed write up! 

Do you need to know this stuff to know more about essential oils? No, but it's interesting, eh?

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